Tapered optical fibre with a reflective coating at the tapered end

ABSTRACT

An optical fibre core ( 39 ) having a reflective coating ( 33 ) along a first part of its length such that electromagnetic radiation may travel along the first part of the optical fibre by means of reflection, and further having a cladding ( 37 ) along a second part of its length, the cladding having a refractive index suitable for permitting the electromagnetic radiation to travel along the second part of the optical fibre. The invention also provides a fibre optic coupling arrangement for coupling a light pipe to a clad optical fibre, the coupling arrangement comprising a light pipe comprising an optical fibre core having a reflective coating and a clad optical fibre comprising an optical fibre core with cladding surrounding the core, the optical fibre core of the light pipe being optically joined to the optical fibre core of the clad optical fibre such that electromagnetic radiation is able to travel from the light pipe to the clad optical fibre, wherein tapered cladding is provided in the region where the light pipe is optically joined to the clad optical fibre.

The present invention relates to optical fibres and particularly thoughnot exclusively to fibre-optic apparatus for detecting electromagneticradiation.

Arrays of fibre optic cables are often used in electromagnetic radiationdetectors. Each fibre has one exposed end so that electromagneticradiation travelling from the direction in which the exposed fibre endis pointing passes into the fibre and then travels along its length. Atthe other end of the fibre is a sensor which may detect, for example,the wavelength or intensity of particular bandwidths of electromagneticradiation, depending upon the desired application, e.g. infra-reddetectors, visible detectors etc. Such detectors are directional in thateach fibre is only able to detect radiation approaching from the generaldirection in which the fibre is pointing.

Therefore it is desirable and usual to have numerous fibres arranged inan array, so that a greater number of directions may be covered by adetector.

Conventional optical fibres comprise a central fibre core surrounded bycladding. Conventional fibre optic arrays comprise a number of cladfibres. The fibres are arranged in a matrix form to create the array,with each fibre being placed as physically close as possible toneighbouring fibres. This permits the amount of ‘dead space’, which isthe space on the array where radiation cannot be guided into a fibre, tobe minimised.

A disadvantage of the known fibre optic arrays is that, due to thecladding, the fibre cores are spaced relatively far apart. Therefore asignificant proportion of the array is ‘dead space’.

This is a particular problem where relatively weak electromagneticradiation needs to be detected. For example, a missile may carry a smalllightweight and therefore relatively weak laser on board forilluminating targets. The missile may also carry an electromagneticradiation detector comprising arrays for receiving the electro-magneticradiation reflected from the target. Because the laser used toilluminate the target is relatively weak, the reflected signals will bevery weak, and therefore it is desirable that as much returned radiationas possible is captured by the fibres of the array.

According to the present invention there is provided an optical fibrecore having a reflective coating along a first part of its length suchthat electromagnetic radiation may travel along the first part of theoptical fibre by means of reflection, and further having a claddingalong a second part of its length, the cladding having a refractiveindex suitable for permitting the electromagnetic radiation to travelalong the second part of the optical fibre.

The reflective coating is preferably a metallic coating. Alternatively areflective polymer material or a semiconductor material may be usedinstead. As the reflective coating only needs to be a very thin layer ofapproximately 100 nm, the coated fibres are significantly thinner thanthe conventional optical fibres previously described. Therefore, thecoated fibres can be packed more densely than the conventional opticalfibres, thus significantly reducing the ‘dead space’ on the opticalfibre array.

In some circumstances during manufacture, eg during drawing of fibres, athin layer of cladding may remain on the fibre core. In suchcircumstances it may be necessary to provide a reflective coating on theouter surface of the cladding to ensure that the electromagneticradiation may still travel along the first part of the optical fibre bymeans of reflection.

Preferably at least part of the outside surface of the cladding iscoated in a reflective coating. The reflective coating is advantageouslypresent in the region close to said first part of the fibre. This helpsto prevent losses of electromagnetic radiation in the region where thefirst, coated part of the fibre and the second, clad part of the fibremeet.

The cladding may be tapered along part of its length, the thin part ofthe taper being adjacent the first, coated part of the fibre.

Advantageously, the tapered portion of the cladding has a reflectivecoating on the outside surface of the cladding. The reflective coatingmay be thickest at the thin part of the taper.

The first part of the optical fibre may have a core of a differentcross-section to the second part of the optical fibre. The coreadvantageously tapers to a larger cross-section in the second part ofthe optical fibre. The first part of the optical fibre may additionallyor alternatively have a core of different cross-sectional shape to thesecond part of the optical fibre. The first part of the optical fibremay additionally or alternatively have a core of a different material tothe second part of the optical fibre.

According to the present invention in another aspect thereof, there isprovided a fibre optic coupling arrangement for coupling a light pipe toa clad optical fibre, the coupling arrangement comprising a light pipecomprising an optical fibre core having a reflective coating and a cladoptical fibre comprising an optical fibre core with cladding surroundingthe core, the optical fibre core of the light pipe being opticallyjoined to the optical fibre core of the clad optical fibre such thatelectromagnetic radiation is able to travel from the light pipe to theclad optical fibre, wherein tapered cladding is provided in the regionwhere the light pipe is optically joined to the clad optical fibre.

The length and shape of the taper may be designed to encourage theelectromagnetic radiation to propagate in a desired mode.

Advantageously, the tapered cladding is at least partially coated with areflective coating. It is particularly advantageous to coat the thinnestregion of the tapered cladding with the reflective coating to preventloss of radiation.

According to the present invention in another aspect thereof, there isprovided an array of optical fibres, each optical fibre comprising anoptical fibre core having a reflective coating along a first part of itslength such that electromagnetic radiation may travel along the firstpart of the optical fibre by means of reflection, and further having acladding along a second part of its length, the cladding having arefractive index suitable for permitting the electromagnetic radiationto travel along the second part of the optical fibre.

The clad part of each of the optical fibres may terminate, for example,in an electromagnetic radiation detection device.

The present invention will now be described by way of example only andwith reference to the following drawings:

FIG. 1 shows a front view of the end of a conventional clad opticalfibre.

FIG. 2 shows a front view of a conventional optical fibre array.

FIG. 3 shows a front view of an optical fibre array in accordance withthe present invention.

FIG. 4 shows a longitudinal cross-sectional view of part of an opticalfibre of the array of FIG. 3.

FIG. 5 shows a longitudinal cross-sectional view of an optical fibre inaccordance with the present invention in a first embodiment thereof.

FIG. 6 shows a longitudinal cross-sectional view of an optical fibre ofa second embodiment.

FIG. 7 shows a longitudinal cross-sectional view of an optical fibre ofa third embodiment.

FIG. 8 shows a longitudinal cross-sectional view of an optical fibre ofa fourth embodiment.

FIG. 9 shows a longitudinal cross-sectional view of an optical fibre ofa fifth embodiment.

FIG. 10 shows a longitudinal cross-sectional view of an optical fibre ofa sixth embodiment.

FIG. 1 shows an optical fibre arrangement 1 comprising an optical fibrecore 3 which is clad in a cladding material 5. The cladding material 5has an appropriate refractive index so that radiation incident on theexposed end of fibre core 3 travels along fibre 1 by means of one ormore guided modes. Some radiation incident on the cladding 7 adjacent tothe fibre core 3 may be coupled into a guided mode. Radiation falling onthe outer part of the cladding 9 will not become a guided mode, and willnot propagate along the fibre 1.

FIG. 2 shows a fibre optic array 11 comprising several optical fibres 1as described with reference to FIG. 1. The optical fibres 1 are packedtogether as tightly as possible. Any electromagnetic radiation fallingon the array has to fall either on the end of the fibre core 3 or theinner part of the cladding 7 to be able to travel along the fibre 1 andtherefore be detected. It can be seen that there is a large area of thearray which cannot be used for detecting radiation, namely the area 13between optical fibres 1, and the area of the outer part of the cladding9 of the optical fibres 1. The ‘dead-space’ (non-detecting) areas are 9and 13.

FIG. 3 shows an optical fibre array 15, which comprises several opticalfibres 17. Each optical fibre 17 has a reflective coating 19 around thefibre core 21. The reflective coating 19 is very much thinner than thecladding, (typically 2-3 orders of magnitude) and allows electromagneticradiation to travel along the fibre core 21 by reflection off thereflective coating.

As the reflective coating is significantly thinner than the cladding, agreater number of optical fibres 17 can be put in an area of the samesize relative to the optical fibres 1 of FIGS. 1 and 2.

This means that the ratio of non-detecting areas 19 (reflective coating)and 23 (area between the optical fibres 17) to detecting areas 21 ismuch lower than for the conventional array 11. As more of the radiationfalling upon the array is received by the optical fibres 17 than for theconventional array 11, more radiation reaches the sensor part of thedetector. Therefore for weak sources of radiation, there is more chanceof detection.

FIG. 4 shows the optical fibre 17. The path 25 of radiation travellingfrom the exposed end of the fibre 21 towards the sensor part of thedetector (not shown) is shown. The radiation is reflected by thereflective coating 19.

FIG. 5 shows an optical fibre 27 comprising a fibre core 29 having anexposed end 31. The part of the fibre core 29 adjacent the exposed end31 is coated in a reflective material 33. The rest of the fibre core 29is clad in a cladding material 35, which has an appropriate refractiveindex. This design of optical fibre allows the exposed ends 31 of aplurality of optical fibres 27 to be packed tightly together into anarray such as that described with reference to FIG. 3 whilst allowingthe rest of the optical fibre to be a conventional fibre waveguide. Thisis advantageous as such waveguides are readily available, and relativelyinexpensive, compared with rare metal light pipes.

FIG. 6 shows an optical fibre 37 comprising a fibre core 39 having anexposed end 41. The part of the fibre core 39 adjacent the exposed end41 is coated in the reflective material 33. The rest of the fibre core39 is clad in the cladding material 35. The cladding material 35 istapered adjacent to the reflectively-coated part of the optical fibre,and the tapered outside surface 47 of the cladding 35 is coated in areflective material 49 which may be the same as reflective material 33.The reflective coating 49 prevents radiation being lost through therelatively thin cladding at the taper 47. Any radiation which reachesthe outside surface 47 of the cladding is reflected back into thecladding 35. The reflective coating 49 may be of constant thicknessalong the length of the taper 47, or may instead decrease in thicknessas the thickness of cladding 35 increases along the taper 47, as shownin FIG. 6. The taper 47 encourages radiation to propagate in desiredmodes. In this example, the fibre core 39 is shown increasing incross-section in the region of the taper 47, however, the fibre core 39may retain the same cross-section throughout the tapered region ifdesired.

FIG. 7 shows an optical fibre 51 comprising a fibre core 53 having anexposed end 55. The part of the fibre core 53 adjacent the exposed end55 is coated in the reflective material 33. The rest of the fibre 53 isclad in the cladding material 35. The cladding material this time is nottapered adjacent the reflectively-coated part of the optical fibre,however the end face of the cladding 57 is coated in a reflectivematerial 59 which may be the same as reflective material 33.

FIG. 8 shows an optical fibre 65 comprising a fibre core 63 having anexposed end 61. The fibre is clad along its length, the cladding 35being significantly thinner near the exposed end 61 of the optical fibre65, and being tapered 69. A coating of reflective material 67 is appliedto the outer surface of exposed end of the fibre 61 and the taper 69).This allows radiation to propagate along the fibre in the region of theexposed fibre end 61 by means of reflection provided that the claddingis sufficiently thin.

FIG. 9 shows an optical fibre 73 comprising a fibre core 75, 77 havingan exposed end 71. The part of the fibre core 75 adjacent the exposedend 71 is coated in the reflective material 79. The rest of the fibrecore 77 is clad in the cladding material 35. The cladding material 35 istapered adjacent to the reflectively-coated part of the optical fibre,and the tapered outside surface 81 of the cladding 35 is coated in thereflective material 79. The reflective coating 79 prevents radiationbeing lost through the relatively thin cladding at the taper 81. Anyradiation which reaches the tapered outside surface 81 of the cladding35 is reflected back into the cladding 35. The reflective coating 79 maybe of constant thickness along the length of the taper 81, as shown inFIG. 9, or may instead decrease in thickness as the thickness ofcladding 35 increases along the taper 81 (as shown in FIG. 6). The taper81 encourages radiation to propagate in desired modes. In this example,the fibre core 75, 77 is formed from two different materials, thematerials joining in the region of the taper 81, and the fibre core 75,77 being designed to allow radiation to propagate along its lengthwithout significant losses in the region of the taper 81. This allowsone material to be used as a lightpipe, and a different material to beused as a fibre core in the clad fibre, where different modes ofpropagation are required. In this example, the fibre core 75, 77 isshown to retain the same cross-section throughout the region of thetaper 81.

FIG. 10 shows an optical fibre 83 having similar characteristics to theoptical fibre 73 of FIG. 9, similar features having the same referencenumerals as in FIG. 9. However, in this example the fibre core 75, 77 isshown to increase in cross-section in the region of the taper 81. Theshape and size of the taper is chosen to encourage propagation of theradiation along the fibre core 77 in the desired modes.

It will be apparent to one skilled in the art that different embodimentsof the present invention are possible, several of which are describedherein. The scope of the invention covers the embodiments which utilisethe same principles as those described herein.

1-14. (canceled)
 15. An optical fibre having a core of which a firstlongitudinal portion is of generally constant cross-sectional area andis covered by an inwardly facing reflective coating to causeelectromagnetic radiation to travel along the first longitudinal portionof the core by means of internal reflection, and a second longitudinalportion is covered by a cladding material having a refractive indexsuitable for guiding the electromagnetic radiation along the secondlongitudinal portion of the core, and wherein the cross-sectional areaof the first longitudinal portion of the core and its associated coatingis less than the cross-sectional area of the second longitudinal portionof the core and its associated cladding.
 16. An optical fibre, as inclaim 15, in which an intermediate longitudinal portion of the core ispositioned between the first and second longitudinal portions, an end ofthe cladding material terminates in the intermediate longitudinalportion, and at least part of the exterior of the cladding material ofthe intermediate portion is covered by an inwardly facing reflectivecoating.
 17. An optical fibre, as in claim 15, in which the end of thecladding material is tapered, and the thinnest part of the taper isdirected towards the first portion of the core.
 18. An optical fibre, asin claim 17, in which the exterior of the tapered end of the claddingmaterial has an inwardly facing reflective coating.
 19. An opticalfibre, as in claim 18, in which the reflective coating is thickest atthe thinnest part of the taper.
 20. An optical fibre, as in claim 15, inwhich a layer of cladding material is positioned between the core andthe reflective coating of the first longitudinal portion, this layer ofcladding material being substantially thinner than the cladding of thesecond portion of the core and sufficiently thin to permitelectromagnetic radiation to travel along the first portion of the coreby internal reflection.
 21. An optical fibre, as in claim 15, in whichthe first portion of the core has a different cross-section to thesecond portion of the core.
 22. An optical fibre, as in claim 15, inwhich the first and second longitudinal portions of the core are formedfrom different materials.
 23. An array of optical fibres comprising aplurality of optical fibres according to any preceding claim, in whichthe first longitudinal portions of the optical fibres are arrangedclosely side-by-side.
 24. An array of optical fibres, as in claim 23, inwhich one end of each of the optical fibres terminates in anelectromagnetic radiation detection device.
 25. A fibre optic couplingarrangement for coupling a light pipe to a clad optical fibre, thecoupling arrangement comprising a light pipe comprising an optical fibrecore having a reflective coating and a clad optical fibre comprising anoptical fibre core with cladding surrounding the core, the optical fibrecore of the light pipe being optically joined to the optical fibre coreof the clad optical fibre such that electromagnetic radiation is able totravel from the light pipe to the clad optical fibre, wherein taperedcladding is provided in the region where the light pipe is opticallyjoined to the clad optical fibre.
 26. A fibre optic coupling arrangementas in claim 25, in which the tapered cladding material is at leastpartially covered by an inwardly facing reflective coating.